Technical Field
The present disclosure relates to a Zener diode.
Description of the Related Art
Zener diodes are commonly used to regulate voltage in a circuit or to supply a stable reference voltage. For this purpose, a Zener diode is reverse-connected in parallel with a voltage source. When the voltage supplied by the voltage source reaches the breakdown voltage of the diode, the latter becomes conducting and then maintains the voltage at this value.
The present disclosure applies in particular to objects having an autonomous low power supply, to communicating objects and to energy harvesting circuits. In these applications, the breakdown voltage of the Zener diode should be as low as possible.
FIG. 1 is a cross-section of a conventional Zener diode formed in a substrate made of a semiconductor material of a first conductivity type, for example P. The Zener diode comprises a well NW having a doping of a second conductivity type, for example N, forming an anode region of the Zener diode. The Zener diode comprises a highly doped cathode region CD of the first P+-conductivity type, formed in the well NW. The region CD is formed on a region ZD having a high doping of the second N+-conductivity type. The regions CD and ZD are isolated from the rest of the well NW by a shallow trench isolation STI. The Zener diode comprises a highly doped anode connection region ED of the second N+-conductivity type formed in the well NW and isolated from the cathode region by the trench STI. Furthermore, the substrate SUB comprises a highly doped region SP of the first P+-conductivity type, forming a bias region of the substrate SUB. The substrate bias region SP is isolated from the regions CD, ZD by the shallow trench isolation STI.
FIG. 2 represents a curve C11 of variation of the current passing through the Zener diode according to the reverse voltage applied between the regions CD and ED. The curve C11 shows the operation of a conventional reverse-biased Zener diode. Between 0 and approximately 2.5V, the current passing through the diode remains low (lower than 10−12 A). From approximately 2.5V and up to approximately 5.2V, the current passing through the diode increases linearly (according to a logarithmic scale) up to approximately 10−8 A. This operating zone which results from a so-called “band to band” phenomenon cannot be used to supply a reference voltage or to perform a voltage regulation. Above approximately 5.2V, a breakdown phenomenon appears, the diode becoming highly conducting, by avalanche effect, while reaching a maximum voltage BV called “breakdown voltage” of approximately 5.5V. The diode keeps this voltage constant irrespective of the intensity of the current, provided that the latter remains between approximately 10−8 A and 10−6 A. Zener diodes are generally used in this operating zone, to supply a stable reference voltage or to perform a voltage regulation.
One proposal already made consists in lowering the breakdown voltage of a Zener diode by thinning the region ZD so as to obtain a transition between the two conductivity types of the regions CD and ZD that is as sudden as possible. However, this solution reaches a limit due to the very low thickness of the region ZD. This solution enables a current to be obtained which increases more rapidly at low voltage values than on the curve C11. The breakdown phenomenon also occurs at a lower voltage than on the curve C11, but the voltage continues to increase according to the current. The “band to band” phenomenon tends to extend over broader ranges of voltage and current values. In these conditions, the Zener diode cannot be used to supply a reference voltage or to perform a voltage regulation.